This invention relates to air moving mechanisms of the type which have particular utility in heat exchange equipment such as water cooling towers. In particular, the invention relates to a unique method and apparatus for adjusting the amount of clearance between the tips of the blades of a large axial flow fan and the inner surfaces of the fan cylinder wall.
In a conventional evaporative water cooling tower of the induced draft type, a single propeller-type fan is rotatably mounted within a fan cylinder to draw air through a fill section positioned therebelow, which air is vertically discharged from the fan cylinder into the atmosphere. Water to be cooled is gravitationally delivered through the fill section in intersecting relationship with the air drawn through the section by the fan. These cylinders generally have a venturi type construction at a point adjacent to and in direct alignment with the plane of rotation of the fan. The inner surface of the cylinder is preferably of a smooth aerodynamically designed configuration. The streamlining of the inner surface of the cylinder wall is desirable in order to decrease friction and air drag and the venturi construction is provided to assure maximum air flow through the tower as well as maximum exhaust air speed out of the tower.
The fans for large capacity cooling towers often are twenty eight feet in diameter or more to move sufficient air to effectively cool the extremely large quantities of water handled by these towers. Manifestly, blades for fans of this diameter must be of sufficient strength to not only withstand cantilever mounting thereof but also resist sufficient deflection during operation. Large cast metal or glass fiber reinforced synthetic resin members of relatively high weight have been found to be the most satisfactory materials for constructing such fan blades.
The provisision of a venturi type fan cylinder for a cooling tower and the advantages derived therefrom is of common knowledge to those skilled in the cooling tower field. In this connection, it is also known that the fan blades must be positioned extremely close to the inner surfaces of the fan cylinder wall in order to take full advantage of the inherent properties of the venturi. In most situations it is desirable that the tips of the fan blades are less than one inch from the inner surfaces of the fan cylinder wall. As one might expect, it has been found to be most difficult to manufacture and install blades and fan cylinders to such close tolerances. The fan cylinder is usually constructed of wood, fiberglass or concrete and is most difficult to manufacture perfectly round. Further, it is most difficult to ensure that the fan is concentrically mounted within the fan cylinder. To the extent that the failure to achieve design tolerances results in the clearance between the tips of the fan blades and the surfaces of the fan cylinder wall to exceed the optimum distance, air is caused to flow over the tips of the blades into a low pressure area beneath the fan. This recirculation of air around the tips of the fan blades reduces the net amount of air moved across the plane of the fan and reduces fan efficiency.
It has been known to compensate for the practical problem of achieving design clearance between the tips of the fan blades and the fan cylinder by securing a one inch thick aluminum honeycomb material bonded to a fiberglass sheet to the surfaces of the cylinder, with the honeycomb material in facing relationship to the fan blades. A roller arrangement is secured to the tip of the fan blade and upon rotation thereof the honeycomb material is crushed into the proper shape to achieve the desired fan blade tip clearance. In practice it has been found that the aluminum material tends to corrode and it is very difficult to apply an effective sealer to the honeycomb material. Further, since the honeycomb material does not conform to the curvature of the fan cylinder, it is necessary to install shims to fill the void spaces. This is very time consuming and expensive.